The present invention relates to video key generators, and more particularly to an improved self keyer that minimizes black content at the boundary between a background video and a fill video.
A self key signal is generated from a fill video that is to be inserted over a background video. The transition between the background video and the fill video has some duration and the edge of the fill video is bounded by black, or the absence of a signal, so that the resulting transition has some black content remaining that is visible and undesirable. This black content artifact is especially noticeable when the luminance of the fill and background videos are similar so that a gap is visually apparent between them. Traditionally when this condition exists, a high gain key signal with a high value clip level is selected in the key generator so that the gap is very narrow and the luminance of the fill video in the gap is high. However this has a rather unpleasant appearance because the edges of the resultant key signal are harsh and any noise in the edges is noticeable.
A conventional self keyer is shown in FIG. 1. The fill and background videos are input to a mixer 10 at ports Y and Z, respectively, together with a key signal, or mixer control signal, at port X. The key signal is derived from the fill video by inputting the fill video to a clip and gain amplifier (keyer) 12 that generates the key signal according to the settings of the clip and gain controls. A delay line 11 is interposed between the fill video input and the mixer 10 to compensate for the processing time of the keyer 12. As shown in FIG. 2 for three different clip and gain settings the output from the mixer 10 has a certain amount of black content at the edges of the fill video.
One proposal for reducing the amount of black content is disclosed in co-pending U.S. patent application Ser. No. 07/325,844 filed on Mar. 20, 1989, now U.S. Pat. No. 4,920,415 by Daniel J. Chaplin entitled "Self Keyer". A second preconditioned key signal is derived from the self key signal and is used to mix the fill video with a gray luminance signal that is adjustable to provide a visual match with the fill video. The result is a smooth luminance transition between the background and fill videos.
What is desired is an improved self keyer that minimizes the black content and the resulting undesirable appearance at the boundary of the transition between background and fill videos without deriving a second key signal from a self key signal.